Grinding ball



Dec. 12,- 1939.

Filed Dec. 15, 1937 lNvENToRs CHA RLES W #AGE/vaucn Fa EDan/cn A. lvls C-oy AfroRNsy I Patented Dec.`12, 1939' PATENT OFFICE GnINDmG BALL l charles w. Hagenbucn and Freuen-aka. Mcooy, Kansas City, -Mo., assignors to Shemeld Steel Corporatiom a corporation of Ohio Application necembe 15, 1931, semi No. 179,935

`s claims.

Our invention relates to grinding elements for use in ball mills, and more particularly toa grinding ball, or slug, for use in such mills..

In' the use of loose members that are adapted 5, to grind by means of a combination of rolling, rubbing and pounding action, such as takes place in the type of pulverizing or grinding apparatus commonly known asa ball mill, various sizes of such loose grinding members, or elements, are

utilized. These Vare ordinarily referred to as halls, even though `the same may notbe true spheres, and are. sometimes referred -to as slugs. The term ball,`as4 used herein, is not-used in 1 in which itl is used in the bau mm art, that is,

to designate one of a collection of loose grinding elements, whichl are permitted to roll and tumble about in a suitable container, such as the drum or other container of a grinding mill.

A Ul

use balls, or loose grinding elements, in the same mill, that 'areof various sizes, either by starting the mill with balls of diilerent sizes therein,

or by obtaining the smaller sized balls, or grinding elements, by the gradual Wear of the elements from a standard starting size. .has been started and is in operation for aperiod of time, there will be numerous Sizes of thev loose grinding elements inthe mill, and it is only necessary to add the largest size elements, that are desirable to use in the mill, as these have to be replaced.

Ball mills are used for many purposes, in reducing materials, and .are adapted for pulverizing many materials. Suchmills are used for grinding materials in' both a wet and aI dry state.

Orcsv and similar materials are usually in-a Wet state.- In grinding materials that are dry, as for instance in the clinker mills inthe manufacture of Portland cement, the materials are usually reduced or ground at a comparatively high temperature, due to the .friction produced -in grinding.

It is accordingly desirable, in making such loose grinding elements for ball mills, commonly 45 referred to asv grinding balls or slugs, toprovide a member that will not wear too rapidly, and

ing balls, or elements, are used in each'ballmill in reducing ores at mines; and for similar pur-` poses, the' fact that these 'carbon-manganese its narrow sense of sphere, but -in the sense It has beenv'founddesirable in b all mills, to-

After a ball mill (cl. 8H)

steel grinding balls of the abovevmentioned character wore rapidly, caused considerable unnecessary expense due to the necessity of purchasing and handling large quantities of these balls, or elements.A above mentioned character is, that when hardcned sufliciently to reducev the rapidity of'wear thereof, itbecomes so brittle, that the tendency of the grinding ball, or element, to crack and ake oi is considerably increased, vand as a e- 10 sult there is no ultimate gain obtained by the A hardening of such a grinding element beyond-a certain stage. While it is desirable to increase the carbon content of steel alloy grinding elements, in order to increase the hardness thereof, yet if the carbon `.content is increased to the higher percentage limit mentioned above 'the proper ductility required for withstanding'impact will notbe present. It has been found by test that the liner the grain of the steel, having a certain hardness, the less the Wear will be under the conditions existing in balll mills. v

While various methods of heat treatment and of hardening have been utilized in the art, and the balls, 'or grinding elements, have been quenched in a-bath of oil, or water, or other cooling medium, to hardenV the same, the effect on the grain structure and Wearingqualities of the heat treatment and rapid cooling of the carbon-manganese steel alloy ball has been such in all cases that lthe life of the balls, or elements, was not as long as is desirable. Variations, of course, exist in the hardness, depending upon the quenching medium, temperatures and compositionszused, but the nalresult, as far as wear is concerned, bythe .use of diil'erent cooling mediums, does not vary greatly, although we have found in practice, in hardening steels of the above mentioned character, that water gives better results, as far as: wear and hardness are concerned, than other cooling baths that might beused.' v

It is a particular object of our invention to .provide a grinding ball, or slug, that 'has very great wear resisting properties, and which is not very much more expensive Vto make than'grinding balls, or slugs, made of carbon steel alloys, previously used for this purpose, so that, d ue to the much Vgreater wearing qualities of our improved grinding element, a-very great saving -in cost'of grinding membersv per tonof material operated on results from the use'of our improved grinding elements; Wehave'found that by us'- ing a molybdenum steel alloy for the grinding. balls, we can use a higher carbon content i n our improved grinding balls or elements than has been previously possible, and obtain a member that has great hardness, great wearing properties, and great resistance to breakage, due to the fact that the alloy de'velopsa` requisite amount The diihculty with-a steel of the 5- utilize. Also, the ball or grinding element made of this alloy will not lose its hardness when used for grinding purposes in cement plant ball mills,

due to the high temperature encountered therein; and materials that tend to adhere to the surface of the usual carbon-manganese steel balls and reduce the efficiency thereof will not adhere to our grinding elements to the same extent as to the carbon-manganese balls previously used,4

thus preventing uneven wear of the balls or grinding elements due to such adherence of the materials that are being ground to the surface of the ball at certain points.

l While it has been known for many years that molybdenum increases the wearing qualities of steel, the molybdenum alloys previously known were not made for successful use in making grinding balls, or slugs, or grinding elements, to use in ball mills. It is, of course, to be appreciated that these brinding balls or elements cannot be made of an extremely expensive alloy, because these a're constantly being used up and have to be constantly replaced in the mill. Even if such molybdenum alloys as were used for other purposes had been used in brinding balls, these balls Awould not have been satisfactory, because either the wearing qualities thereof would not have been suicient to make the use thereof economical, or`

the first cost of these balls would have beenl so great that the increase in the wearing properties thereof would have been insufficient to overcome this increase in cost. An entirely different situation exists in making grinding balls than in making tools, or railroad rails, or other similar members, that are used for long periods of time. Grinding balls of the Vmost excellent wearing qualities could not have an extremely long life, because of the hard service to which the same are subjected. The wearing qualities of alloy steels, in which molybdenum has been previously* used, have been attributed to the resistance to abrasion of the molybdenum itself.' From this standpoint the greater the percentage of molybdenum used the greater the wearing properties of the alloy. For this reason it had been, prior to this invention, customary to keep the carbon content of molybdenum alloys relatively low and the molybdenum content relatively high, in order to increase the wearing properties, and obtain toughness. j

We have discovered, however, that in a grinding ball or grinding element, molybdenum in the alloy used can serve an entirely dierent purpose. The presence of the molybdenum increases the wearing properties of the alloy, but it is not primarily due to the abrasion resisting properties of the molybdenum' itself that increased wear and longer life in our grinding ball is obtained. 4The increase in wear and longer life is due to the fact that, by the use of molybdenum, we are able to control the grain structure by heat treatment and hardening ofthe balll or grinding element,

whereby we obtain a denser and nner grained steel than has been previously obtained in grinding balls. The nner grain structure that is obtainedis coupled with a greater depth of hardvenlng than has been previously obtainable in hardened steel grinding balls. As a result, a ball is produced that is hard and does not wear rapidly on its surface, and is not of a coarse grain structure, and yet is not so brittle that it will chip oif or crack underimpact, such as exists under conditions present in many ball mills.

It is a particular purpose of our invention to provide a grinding ball of the above mentioned character that is made. up of steel of veryfine grain structure, in the martensitic state, which extends inwardly from the outer surface of the grinding ball or element to the center thereof in the case of small grinding balls or elements, and inwardly to substantially the center thereof in grinding balls of medium size, while in the case of grinding balls of the larger sizes, the fine grained martensitic structure of the steel extends inwardly to such a depth that the size of the grinding elementor ball will have been reduced, in even the largest size balls, by substantially one-third the original diameter thereof, before the inner core of a coarser grain structure is reached. The inner core is, however, of a ner grain than is obtained by heat treatment and quenching of previously used carbon-manganese steel alloys, no matter what sort of heat treatment and cooling medium might have been used in carrying out the hardening process. The center of the core is largely troostite and sorbite, While an intermediate zone between the outer ne grained martensitic -zone`and the center of thecore, comprises martensite, as well as troostite and sorbite, the core having more elasticity than the outer hard, dense, ne grained zone.

It is also a purpose of our invention to provide a new and improved method of making a grinding ball or element, whereby a grinding ball or element .having the aforementioned desirable characteristics is produced. 1

Other objects and advantages of our invention will appear as the description of the drawing proceeds. We desire to have it understood, however, that we do not intendto limit ourselves to lthe particular details shown or described, except as defined in the claims.

In the drawing:

' Fig. 1 is a transverse sectional view through a grinding ball of relatively large diameter, made in accordance with our invention, showing the grain structure, and

Fig. 2 is a similar sectional view of a small grinding ball, made in accordance with our inventlon.

In making our improved grinding member, a

molybdenum steel alloy is used, which, preferably, contains carbon, manganese, molybdenum, and silicon or aluminum, within the percentage limits below set forth:

- a Per cent Carbon .60 to 1.10 Manganese l .30 to 1.1 0 Molybdenum .10 .to .80 Silicon or aluminum Up to .30

As a specific example ofv an alloy found highly' desirable and giving'thedes'ired grainstructure,

hardness and wearingproperties for a u v ball of one-half inch diameter,we have found the pletely immersed in the water for ve seconds,` the water being at a temperature between80 and following composition to be preferred:

Per cent CarbonJ 1 .70 Manganese .60 Silicon, not over: .25 Molybdenum 20 Iron, not less than 98.00

Impurities', mainly sulphur and phosphorus,

approximately .25

The grinding element lis formed in any desired -manner into the shape desired, which may be 'grind element attains the desired temperature it is quenched in a cooling medium for a suflicient -period of time to reduce the temperature of the grinding ball to approximately 200 degrees F., after which it is cooled, in the atmosphere, to atmospheric temperature at a rate of not to exceed 200 degrees F. per minute. 'I'he quenching can be done in water, oil, or brine, as may be found preferable. When a ball one-halfvinch in diameter is quenched in weten-the ball is come 90 degrees F. Instead of heating thel grinding ball after it has been forged to shape, it may be f quenched in the quenching medium, in the case of small sized balls, immediately after forging the same, before it cools,without re-'heatingbut in that case, the manganese content of the ball is,

preferably, reduced below that above mentioned.

- While the composition above gnentioned is preferred for a ball one-half inch in diameter that is quenched in water, some variation from this composition can be used and yet approximately the same results can be gotten as with the preferred composition. Thus. the carbon content can be life-that form fthe principal advantages -of our improved grinding elements'. ofcourse, if it is desired to produce ai rust or corrosion resisting vadditional strength by means suovyl v f t As the diameter of the balls'increases, it is 'ball from .20% to .30% copper can be addedjv to the composition, or even higher percentages of copper can be added, if it is desired to not onlyA provide rust or corrosion resisting properties, but of copper-in the found to be desirable tc-increase the V.carbon content to :obtain these desirable characteristics.'v Thus, forexample, a yball three-fourths inch inv diameter, preferably', contains from .60% to .9,0% carbon; aone inchball, preferably, contains from .62% to .95% carbon; a one androne-halfk inch ball from .62% izo-3.95% carbonya two incl ball i from .62%l to 1.00% carbon;.a"three inch ball from .65%- 'to 1.00% carbon.; a four inch ball from .70% to'1.05% carbon; a five inch ball from .70%

to 1.05% carbon, whilea ball teninchea' in iiiameter will, preferably, have from .75% to 1.10% carbon.

'I'he character of the quench used determines to a certain extent the amount of carbon that should be present in the alloy. Thus a brine quench will require the lowest percentageof carbon-and anv oil quench the highest percentage. Thus for the one-half inch balls, preferably, the carbon content of-a ball that is to be oilqnenched is `from .75% to 35%; for a ball to be water quenched,l from .65% to .75%, Vand for a ball to be brine y quenched, from .60% to .70%.

The temperature to which the ball vis heated before quenching is also varied in accordance with the quench used. 'I'hus for a brine quench the temperature at which the ball is quenched is from M25-degrees F. to 1450 degrees F., whereas for a water quench it is from 1450 to 1500 degrees F., and for an oil quench, from 1500 to 1550 degrees F., for a one-half inch ball.n The quench# ing temperature also gradually rises with the size of the ball, and for a ve Vinch ball the quenching temperatures are' substantially 125' degrees F. to 175 degrees F. more' than for a onehalf inch ball. Also the quenching time required increases gradually with the size of the ball. For example, when a water quench is used the time required for quenching the one-half inch ball is five seconds; the three-fourths inch ball ten seconds; the one' inch ball eighteen seconds; the two inch ball forty-two seconds, the

three inch ball eighty seconds and the ve inch ball one hundred and ten seconds.

The manganese content of the balls varies with the size of the ball and also is dependent ,upon the re-heated and then quenched a higher percentage of manganese can be-used without causing internal stresses in and brittleness of the balls, than if 'content varying between .50%. and .60%, and a three inch water quenched ball. similarly treated, a manganese content varying-between.55% andA .75%, and an oil quenched ball of one-halflinch diameter, similarly treated, may have a 'man' ganese content varying between .55% and .75%, a

one inch oil quenched ball, similarly treated, a

manner in which quenchingfis done. If the balls are allowed to cool in the atmosphere and are manganese content varying between .65%,I and .85%, and a three inch oil quenched ball a manganese content varying'b'etween.80% and 1.00%.

' If a brine quenchis used, the ball having been` similarly treated. the manganese content of a one- .45% massa. and. a mennen in 'diameter bau `half inch inV diameter ball'may vary between .40% land .60%; a Vone inch in diameterball between foo betweeri.50% Vand .70%. 1 If the ballsare'directly water quenched Yafter forging, the manganese contefnt' may vary between .35% and 155% fora bally l 05' .60% for a ball -onexinhfin diameter-'and be-A one-'half inch in diameter-ruimen .40% and tween .45% to .65% :for a ball f inches j in diameter. If the balls'aremil quenched .directly l brine quenched directly lafter forging., the .man-

'ganese contentmay vary between .30% :and .50%

'. after.forging', the manganeseqfccntent'may vary`- betweenv .45% and %!Qra ball one-half inch in diameter; between .50% andk70% fora-ballena' all the smaller size balls, no matter what type of t quench is usedup to and including two inch in diameter balls. Balls up to'and including those of five inch diameter, preferably, contain a somewhat higher percentage of molybdenum, varying between the limits of .15% and .25%. In larger sizes than ve inch balls the molybdenum content may be increased, so that for anyquenching medium a molybdenum content of .40% `to .60% is preferable for balls of Aten inch diameter, for

example,V and may run as high as .80% for the 1 larger size balls.

The silicon content, preferably, does not vary to any extent for the different size balls, being, preferably, kept between .10% and .20% for all size balls where a brine quench is used, or where a water quench is used, and between .15% and .25% for all size balls up to and including five inch in diameter balls, where an oil quench is used, and between .20% and .30% for balls larger than flve inch in diameter, where an oil quench is used.

Of course, the percentages of manganese and molybdenum may be varied from the ones stated herein, as well as the percentage of silicon, where balls withspecial characteristics may be desired, as long as the variation is not too great from the extreme percentage limits given. Also, forextremely large size balls it may even be found desirable to vary the content of molybdenum and4 manganese above the upper limits stated.

The grinding element that results from the use of the alloys substantially as set forth, treated in the manner. described herein, varies somewhatin 'characteristicadependent upon the sizeof the ball, slug, or lgrinding element, that is produced. In all cases a very hard :ne grain structure is produced in the steel inwardly from the outer surface of the grinding ball, or element, a very substantial distance. This outer zone is of a lineness of grain and hardness, that has not been previously attained in steel grinding balls of any kind. This hard, fine grained structureextends to a depth of at least one-half inch from the outer surface of the ball, even in the larger size balls. and extends to the center of the ball in the smaller size balls, up to and including a ball of one inch diameter. The structure of this hard, ne grained portion of the larger balls and of the entire ball, in the case of balls of, one inch or smaller in diameter,is a ne Vgrained martensitic A50 structure.

While this nue has extremely great resistance to abrasion and great hardness 'and some ductility, vit does not have suiilclent ductility to withstand the impact,

'mi that results when larger balls are dropped in the course of their travel in the ball mill, which, in`

mills ipeyvhich larger balls are used, is often a conr siderblniirop from the upper portion of the mill .t- ,to the er portion thereof. In order thatthis y be absorbed in the ball without any "dan fracture of the ball, it is deslrablethat fthe fl' er grinding elements for use in ball mills i avea-core of different structurethan thel outer portion thereof, and one of the advantages of our improved grinding element, due to the alloy used grained martensitic structure l and the method of treatment of the same to harden it, is that, in the larger sizes, by simple heating and quenching, the desired core of a different structure is obtained to absorb the impact resulting from the dropping of such larger balls and the prevention of fracture of the balls thereby, while at the same time providing the martensitic structure of a deep outer layer of the ball, or grinding element, which is necessary to get the long wear that makes our improved grinding element highly desirable and economical.

Referring to the drawing, Fig. l shows as adequately as is possible by means of a drawing, the internal structure of a larger size grinding ball, made in accordance with our invention, and Fig. 2 shows the internal structure of a smaller size grinding ball, made in accordance with our invention. The type of grinding ball shown in Fig. 1 is that of the grinding balls larger than one inch in diameter, and that shown in Fig. 2 of grinding .balls of one inch in diameter and smaller.

'Ihe ilneness of the grain of the grinding ball is necessarily shown somewhat diagrammatically, but referring to the grinding ball shown in Fig. 1, a zone or layer of hard very fine grained martensitic steel is shown at 3, extending from the outer surface 4 of the ball to a fairlywell defined point 5, spaced inwardly substantially a uniform distance from the outer surface 4, at which a coarser structure' begins, which extends to the enter 6 of the ball. This coarser structure' is made up of troostite and sorbite, and is of considerably coarser grain than the outer martensitic structure 3. The inner core portion, designated generally by the numeral Vl, extending from the center 6 outwardly toy substantially the point i, is, however, finer grained lthan any portion of a hardened carbon-manganese ball, except, possibly, a very thin skin-coat adjacent the outersurfaces of such a carbonmanganese steel alloy ball. It has no crevices or cracks therein of a decided character, as are i found in a carbon-manganese vsteel ball, both in the hardened outer thin skin-coatlike zone and the inner portion thereof, which is of very coarse grain, as compared with the coarsest grain portion of the core 1. .There is an intermediate zone in the larger size balls, such as shown in Fig. 1 of the drawing, that contains a mixed structure that appears to be partially of the character of the inner portion of the core 1 and partially martensitic, like the outer hard fine grained portion lf This zone is not of any great depth, but consists of martensite and troostite, or martensite, troostite and sorbite.

The presence of the molybdenum in the alloy appears to be responsible for thefact 'that a.

grinding ball of eithery the character shown in Fig. 1, or the character shown in Fig.- 2, will notl lose its hardness 'and fine grain structure, if usedl under relativelyhigh temperature conditions. Figl 2, of course, shows a grinding element that is made lup of the fine grain, hardened martensitic structure 3 from the outer surface l thereof to the center 9 thereof, being of av homogeneous character throughout.

The greater percentage of carbon is provided in the larger size balls, in order to robtain the desired hardnessl of the outer zone I thereof.

The point atwhichthe change from beta to gammairon, or vi'oe (versa, takes place, is, of course, raised by increasing theV molybdenum content, and this is desirable for larger size grinding members.l as the temperature to which fore quenching can thus be increased to obtain a more thorough solution of the carbon, manganese and molybdenum in the iron, and atthe same time obtain the desired ilne grain structure. The quenching time is increased as the size of the balls increases, to permit withdrawal of the larger grinding elements at substantially the same temperature as the smaller ones, this being, of course, necessary because v,of the larger mass of the larger balls.

We have found that ballsup to two inches in diameter may be cooled slowly in the air after quenching to obtain the desired rate of cooling, while those that are of al diameter above two inches, and including those four inches in diameter, are run into a furnace that is operated at a temperature of 400 degrees'F. immediately after quenching, tovprevent too rapid cooling, being preferably, kept in such furnace for about half an hour, after which the same are slowly cooled in the air. Balls of larger diameter than Y those one inch in diameter are of the same hardfor extremely small grinding elements, than the ordinary carbon-manganese alloy grinding balls that have been previously used. A comparison between forged carbon-manganese grinding balls and our molybdenum alloy grinding balls, also forged, and both made according to` the best known practice, and similarly heat` treated and quenched, shows, upon hardness tests'v being made of said balls at the surface thereof and at the center thereof, that balls up to and including ness in the. center as at the wearing surface, both when made of the carbon-manganese alloy steel previously used, and when made by our improved method of the molybdenum alloy steel that vwe employ','but that the hardness of the balls made in accordance with our invention isgreater for `all sizes of balls than the carbonmanganese alloy balls,lv and that the largerthe ball the greater the diiference. lI `hus, for a onehal-f inch ball the carbon manganese steel ball cente thereof, while our improved grinding element as av hardness at the surface and center thereof of 65'Rockwell C scale, or '720 Brinell.'-

A one inch ball of carbn-manganese steel alloy' v has a Rockwell C scale hardness of 63 both at the v scale at both thesurface and' the center thereof,

surfaceL and fthe center thereof, or680 Brinnell, while la one inch-'ball made in accordanceA with our' invention hasA af; hardness of 66 Rockwell C or '75g-Brinell. Y."

The use of o does. not vary to'any extent from that -atthe surface `until Aconsider-ably larger 'sizes' are-enf countered, whilethe hardness` of ai carbon- 'ganese steel very perceptibly 'in the center, Aaswellas on. the' surface `thereof afterfthe diameteri. increases above oneinchl Thus atwo' inch stljgrinding ball has a hardness of only 58 Rockwell C Y scale, or.610 Brinell, at the surface thereof, and

a hardness of only 38 Rockwell C scaleor 370 Brinell at the center thereof, whereas our im-4 proved ball of the alloy herein described, two inches inl diameter, has a hardness at the surface of 64 Rockwell C scale, vor 700 Brinell, and 50 Rockwell C scale or 510 Brinell at the center. A three inch ball of carbon-manganese steel alloy has an outer surface hardness of 56 Rock- 4well C scale, or 580 BrinelLand a center hardness of 37 Rockwell C scale, or 360 Brinell, whereas a ball bf three inches in diameter made in accordance with our invention has a surface hardness of 63 Rockwell C soale,.or 680 Brinell, and a center hardness" of 46 Rockwell C scale,

or 460 Brinell.

.It will thus be noted that while the center of the three inchball is considerably less hard than that ofthe two inch ball in the case of our im-4 proved grinding ball, Vthe surface hardness is substantially the same, and this hardness extends inwardly a`very substantial .distance from the surface, whereas in the'case of the carbonmanganese alloy it doesv not extend inwardly any appreciable distance, but the hardness begins decreasing rapidly` from vthe outer surface towardthe center of the ball. While the difference in hardness between ra ball made in ac'- cordance with our inventionv and the ordinary carbon-manganese steel balls is vnot as great for the larger. s'izeballs as lfor those between one inch and three inches in diameter, yet the larger size balls are harder both'on the surface and at the center thereof, when our improved molybdenum alloy steel and method of treatment is used, than when the ordinary carbonmanganese steel is used.- Thus a -ive inch ball .of carbon-manganese steel has anexteriorhardness of 50 RockwellV C` scale, or 510 Brinell,

and a center hardness o f 30 Rockwell C scale, or

300-Brinell, while a five inch ball made in accordance with our invention has a surface hardness of 53 Rockwell C scale, or 540 Brinell, and

a center hardness of 40 Rockwell C scale, or

390 Brinell. l

Theaverage hardness, obtained by testing at Vregular, spaced lintervals between the outer surface and the center,'ofballs made in accordance with our invention and'in accordance with previous practice for carbon-manganese steel grinding balls, shows that our: improved grinding has a Rockwell C scale hardness of, 64, orV A'700 Brinell, at the surfacethereof, and also at .the

ball for all'sizes, has: an average hardness, from Y theV outer 'surface to Y the` center.' thereof, that is much greater than thatof the'carbon-manganese steel ball.- Thus'a ball-of one-half inch diameter of carbon-manganese steel has ariaverag" hardness of 641'Rockwell1C scale, or 700 Brinell,. and our improved'f-ball v'of .the same diam'- eterhas an average hardness'5of'65gRockwell C scale, tir-726 Brinell. ,'A-'oeinchjball of carbonmanganese'steel hasan'avera'ge-vhardness of 63 Rockwell C scale or' 6,80'Brinellgvltwhile1our im- Y Vproved ball of the sameszehas anl average hardness,.of 66 Rockwell C1 scale, jor-'750 Brinell. A two inch carbon-manganese :ball .has an .averageha'rdness of 53 Rockwell ICscale, or 540 YBrinell, while a two inch Vball of'our manufac- .turehas an'v averagehardnessof 63 Rockwell C :scale Aor 6 80 Brinell. The ,three linch carbonmanganese'steel ball'hasv an averagefhardness ball vof' our ina'nufacture,'threeinchesv ln diameter, has anaverage hardness of 624 Rockwell C' manganese steel ball has an average' hardness of 36 Rockwell C scale, or 350 Brinell, and our improved ball five inches in diameter vhas an average hardness of 43 Rockwell C scale, or 420 Brinell.

It is obvious from the above that we have provided aball that not only has greater average hardness, but has through a large portion of its depth, a much finer grain structure than any previously manufactured steel grinding ball, and that, as a result, the wearing properties of 'these balls is mudh greater than that of steel grinding balls previously used for ball mills and similar apparatus. 'I'he effect of having the finer grained portion of considerable depth, has a very great eiect on the wearing qualities ofthe balls. As the depth of the fine. grained, substantially homogeneous outer zone is at least one-half inch, the fine grained martensitic wearing zone of a ball two inches in diameter comprises about 487.5% of the entire mass of the ball, and in the case of a three inch ball '10.4% ofthe entire mass of vthe ball, while in thecase of a four inch ball `it would comprise 57% of the entire mass of the ball, and for a five inch ball 48.8% of the entire mass of the ball. Comparing this with carbon-manganese steel balls, which do not have the fine grain structure, even to a depth of onefourth inch on an average, but assuming that such maximum depth is reached by the finer grain structure in the carbon-manganese steel ball, this finer grained zone only comprises 57.8% of the entire mass of the ball in a two inch ball; only 42.1% of the entire mass in a three inch ball; only 33% of the entire mass in a four inch ball, and only 27.1% of the entire lmass in a ve inch ball. The Wearing qualities of a ball made in accordance -with our invention' are accordingly increased because of this larger zone of such dense ne grained, hard, long wearing.' steel, and are' additionally increased above that accountable to'this fact.` because of the 'greater hardness and nner grained and denser structure of the inner portion of the larger balls, further because of the intermediate zone of considerable depth that'is largely fine grained martensitic steel, and also because of the fact that there are no interior cracks or crevice: of any character in our improved grinding elements, such as are found in carbon-manganese steel grinding elements previously known.

What we claim is:y p

1. A steel grinding element for ball milla-constituting a solid lball-like body ofl heat' treated carbon-manganese-molybdenum steel alloy hardened throughout from its outer surface "to its` center of mass and having a 'deep wearing zone of fine grained structure extending from the outer wearing surface inwardly in asolid mass toward the geometrical center thereof, and a central solid core o'f coarser `grained structure surrounded by and integral with lsaid wearing zone, said alloy containing .60% to 1.10% car-- bon, .30% to 1.10% .10% to .80% molybdenum, not more than .30% impurities and not less than 96.'l%'iron.

2. A steel grinding element for ball mills, constituting a solid ball-like body of heat treated,

carbon manganese -modybdenum steel alloy hardened throughout from its outer surface to its center of mass. andv having a deep wearing zone of fine grained extending .from the outer wearing surface inwardly in a solid mass toward the geometrical center thereof, and a central solid core of coarser grained structure surrounded by and integral with said wearing zone, said alloy containing .60% to 1.10% carbon, .30% \to, 1.10% manganese, .10% to .80% molybdenum, not more than .30% impurities, .20% to .30% copper and notless than 96.4% iron.

3. A wear resistant, heavy steel solid ball oi heat treated, carbon-manganese-molybdenum steel alloy hardened throughout from its outer surface to its center of mass and having a deep outer, hardened wearing zone of fine grained martensitic structure, and a central, solid, hardened core of coarser grained structure integral with said wearing zone, said ball having a zone between said core and outer zone of mixed fine and coarser grained structure. said alloy containing .60% to 1.10% carbon, .30% to 1.10% manganese, .10% to. 80% molybdenum, not more than .30% vimpurities and not less than 96.7% iron.

4. A ball-like carbon-manganesemolybdenum steel alloy grinding element for ball mills, constitutinga solid body hardened throughout its mass, having a deep zone of hardened fine grained'martensitic structure extending from the louter wearing surface inwardly in a solid mass .f

toward the geometrical center thereof, and a central solid hardened core of vsorbitic and troostitic structure havingagreaterelasticity than said fine grained zone, surrounded by and integrally connected with said fine grained outer zone by a zone of mixed martensitic, sorbitic and troostitic structure, said core being of less volume than said fine grained martensitic zone, said alloy containing .60% to 1.10% carbon, .30% to 1.10% manganese, .10% to .80% molybdenum. and the balance principally iron.

5. A wear resistant heavy carbon-manganesemolybdenum steel alloy solid ball hardened throughout and having a deep outer zone of hardened fine grained martensitic structure extending from the outer wearing surface inwardly toward the geometrical center thereof; and a central solid hardened core of sorbitic and troostitic structure having greater elasticity than said fine grained zone integral with said fine grained zone, the grain and hardness of said steel alloy changing gradually through .a zoneV of mixed martensitic, sorbitic and troostitic structure between said inner and outer zones..said alloy containing .60% to 1.10% carbon, .30% to 1.10% manganese, .10% to .80% molybdenum, and the balance principally iron.

6. An integral ball-like carbon-manganesev molybdenum steel alloy grinding element for ball mills, solid from the geometrical center thereof to the wearing surface thereof and hardened throughout its mass, said wearing surface extending over the entire periphery of said ele- -A ment and being uninterrupted, said element being of a hardened fine grained martensitlc structure inwardly from'all portions of the peripheralcnanfns w. HAGENBUCI'I.' FREDERICK A. MccoY. 

